Genetic studies have found that Huntington's Disease (HD) is caused by an abnormally elongated polyglutamine (polyQ) tract in the large protein huntingtin (htt). However, both the normal function of htt in neurons and the molecular mechanism by which the expanded polyQ in htt causes selective dysfunction and degeneration of striatal GABAergic medium spiny neurons (MSNs) remain elusive. Emerging evidence suggest that polyQ-htt may in part mediate its neurotoxic action in HD by altering neuronal membrane trafficking and synaptic function. Several htt-interacting proteins implicated in intracellular transport have been identified, one of which is huntingtin-associated protein 1 (HAP1). It has been found that HAP1 interacts with GABAARs, as well as kinesin or dynein microtubule motor proteins. It is our working hypothesis that polyQ-htt, via tighter binding to HAP1, interferes with the interaction between GABAARs and kinesin motor proteins, which leads to altered membrane trafficking of GABAARs along dendritic microtubules and diminished inhibitory synaptic transmission at the early stage of HD. To test this, combined electrophysiological, immunocytochemical and biochemical approaches will be used to address two specific aims. (1) To study the altered GABAAR trafficking in HD. The inhibitory synaptic strength and surface expression of GABAARs will be compared in striatal MSNs from WT vs. HD mice or transfected with WT-htt vs. polyQ-htt. (2) To study the molecular mechanism underlying the altered GABAAR trafficking in HD. The role of a multiprotein complex containing htt, HAP1, KIF5, and GABAAR in inhibitory synaptic responses will be examined. Alteration of the GABAAR/KIF5/microtubule complex will be examined in mouse models of HD. This proposal will for the first time reveal the altered GABAAR trafficking and synaptic inhibition in HD, and shed light on how polyQ-htt disrupts the HAP1/KIF5-dependent delivery of functional GABAARs to synaptic membrane. The diminished strength of inhibitory synaptic transmission in striatal MSNs could contribute to the loss of the excitatory/inhibitory balance, leading to increased neuronal excitotoxicity. Understanding the molecular and cellular mechanisms will provide valuable targets for designing novel therapeutic target to restore altered inhibition in Huntington's disease. PUBLIC HEALTH RELEVANCE: This study aims to reveal the altered GABAAR trafficking and ynaptic inhibition in Hungtington's Disease (HD), and shed light on how polyQ-huntingtin disrupts the HAP1/KIF5-dependent delivery of functional GABAARs to synaptic membrane. The diminished strength of inhibitory synaptic transmission in striatal medium spiny neurons could contribute to the loss of the excitatory/inhibitory balance, leading to increased neuronal excitotoxicity. Understanding the molecular and cellular mechanisms will provide valuable targets for designing novel therapeutic target to restore altered inhibition in HD.